docs: def _compute_phi_ss(C, mag, c1_dists, log_phi_ss, mean_phi_ss):
      """
      Returns the embeded logic tree for single station sigma
      as defined to be used in the Swiss Hazard Model 2014:
      the single station sigma branching levels combines with equal
      weights: the phi_ss reported as function of magnitude
      as proposed by Rodriguez-Marek et al (2013) with the mean
      (mean_phi_ss) single station value;
      the resulted phi_ss is in natural logarithm units
      """
  
      phi_ss = 0
  
      if mag < C['Mc1']:
          phi_ss = c1_dists
  
      elif mag >= C['Mc1'] and mag <= C['Mc2']:
          phi_ss = c1_dists + \
              (C['C2'] - c1_dists) * \
              ((mag - C['Mc1']) / (C['Mc2'] - C['Mc1']))
      elif mag > C['Mc2']:
          phi_ss = C['C2']
  
      return (phi_ss * 0.50 + mean_phi_ss * 0.50) / log_phi_ss